1. Field of the Invention
The present invention generally relates to detecting defects on a wafer.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as ICs. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
One obvious way to improve the detection of relatively small defects is to increase the resolution of an optical inspection system. One way to increase the resolution of an optical inspection system is to decrease the wavelength at which the system can operate. As the wavelength of inspection systems decreases, incoherent light sources are incapable of producing light with sufficient brightness. Accordingly, for inspection systems that are designed to operate at smaller wavelengths, a more suitable light source is a laser light source that can generate relatively bright light at relatively small wavelengths. However, laser light sources generate coherent light. Such light is disadvantageous for inspection since coherent light can produce speckle in images of a wafer. Since speckle is a source of noise in the images, the signal-to-noise ratio (S/N) in images generated by inspection systems will be reduced by speckle. In addition, speckle noise in wafer inspection systems (e.g., laser-based inspection systems) is one of the main limitations of defect of interest (DOI) detection ability. As wafer design rules continue to shrink, optical inspection systems preferably have shorter wavelengths and larger collection numerical apertures (NAs). Speckle noise consequently increases to a more dominant noise source.
Many illumination systems have been developed for inspection applications that reduce the speckle of light from laser light sources. For example, popular approaches to reduce speckle noise currently involve reducing coherence of the illumination laser source by transmitting light through an optical diffuser or vibrating optical fiber. These approaches usually require increasing the illumination NA on the wafer and therefore are not effective for an outside-the-lens (OTL) oblique angle illumination architecture. Reduction of laser coherence also limits the usage of Fourier filtering and degrades the S/N. Other approaches such as moving an aperture in the pupil plane have been applied to select a spatial sample of light in the pupil plane and then average the image over a relatively large number of samples. This approach will greatly reduce the resolution of the optical system thereby decreasing the defect capture rate.
Some methods for defect detection utilize output generated by multiple detectors of an inspection system to detect defects on a wafer and/or to classify defects detected on the wafer. Examples of such systems and methods are illustrated in International Publication No. WO 99/67626 by Ravid et al., which is incorporated by reference as if fully set forth herein. The systems and methods described in this publication are generally configured to separately detect defects in the electrical signals produced by different detectors. In other words, the electrical signals produced by each of the detectors are processed separately to determine if each detector has detected a defect. At any time that a defect is detected in the electrical signals produced by one of the detectors, the electrical signals produced by at least two of the detectors are analyzed collectively to determine scattered light attributes of the defect such as reflected light intensity, reflected light volume, reflected light linearity, and reflected light asymmetry. The defect is then classified (e.g., as a pattern defect or a particle defect) based on these attributes.
Although the methods and systems disclosed in the above-referenced publication utilize scattered light attributes of defects determined from electrical signals generated by more than one detector, the methods and systems disclosed in this publication do not utilize electrical signals generated by more than one detector in combination to detect the defects. In addition, the methods and systems disclosed in this publication do not use a combination of electrical signals generated by more than one detector for any defect-related function other than classification.
Other currently available inspection systems are configured to inspect a wafer with more than one detection channel, to detect defects on the wafer by separately processing the data acquired by each of the channels, and to classify the defects by separately processing the data acquired by each of the channels. The defects detected by each of the individual channels may also be further processed separately, for example, by generating different wafer maps, each illustrating the defects detected by only one of the individual channels. The defect detection results generated by more than one channel of such a system may then be combined using, for example, Venn addition of the individual wafer maps. Such inspection may also be performed using output acquired in a single pass or multiple passes. For example, one previously used method for defect detection includes performing two or more scans of a wafer and determining the union of the lot results as the final inspection result for the wafer. In such previously used methods, nuisance filtering and defect binning is based on the Venn ID results, AND/OR operation, from multiple scans.
Such previously used inspection methods, therefore, do not leverage the output generated by the inspection system at the pixel level, but rather combine the results at the wafer map level as the final result. Defects are detected by each pass independently based on their relative signal (magnitude) compared to the wafer level noise seen for each pass. In addition, nuisance filtering and binning in previously used methods may be based on the AND/OR detection from multiple scans and thereafter separation in each individual scan. As such, no cross-pass information other than the AND/OR operation on detection is considered.
Accordingly, it would be advantageous to develop methods and systems for detecting defects on a wafer that combine information from different optical states of an inspection system to increase the S/N of defects in image data for the wafer used for defect detection while decreasing noise (e.g., speckle noise) in the image data.